(a). Field of the Invention
The present invention relates to a method for data transmission rate adaptation, more particularly, to a rate adaptation method for Wideband Code Division Multiple Access (WCDMA).
(b). Description of the Prior Arts
Along with the progress of technology, the use of cellular phone becomes more and more popular and, as the result, the demand for higher performance in cellular phone is increasing. That is, besides the basic voice communication, owing to the popularization of the Internet, the cellular phone users also desire to be able to use his cellular phone for data transmission. Consequently, the demand for higher data transmission rate is increasing
Under the same surrounding, a cellular phone with higher transmission power can achieve a higher transmission speed. However, the transmission power of a cellular phone is limited. Moreover, a cellular phone with high transmission power is prone to create interferences to its neighboring users and, in consequence, forces the neighboring users to increase their transmission power which create interferences in return. Accordingly, a positive feedback effect similar to the armament race is generated. Hence, it is obvious that to increase the transmission power is not a good idea for increasing transmission speed. In addition, the increasing of transmission power also increases the power consumption that reduces the battery usage time and causes inconvenience for users.
In fact, environment is a major factor considering the quality of radio communication. Including path loss and noise interference can have influence on transmission power for better communication quality. Therefore, in order to have a satisfactory communication quality, it is inclined to increase the transmission power or to reduce the transmission speed for reducing bit error rate when in worse communication environment, on the contrary, it is inclined to reduce the transmission power or to increase the bitrate for increasing throughput. Hence, in the Third Generation Partnership Project (3GPP), A principle specification is being made for adaptive power and rate jointly. Please refer to FIG. 1, which is a guideline for adaptive power and rate jointly in 3GPP. The base station using 3GPP will set a limit to the transmission power of each cellular phone, that is, the transmission power of cellular phone is restricted within a working range [the maximum allowable transmission power, the minimum allowable transmission power]. The cellular phone will search an appropriate transmission rate with in this working range. If the transmission power of cellular phone is larger than the maximum allowable transmission power, the situation represents that the transmission environment is worsening and requires a larger transmission power to maintain the data transmission rate. However, the adequate response now is to reduce the data transmission rate, rather than increase the transmission power. As aforementioned, the increasing of transmission power will result in a worse interference to the other users and, as such, will also result in a disadvantage to oneself. If the transmission power of cellular phone is smaller than the minimum allowable transmission power, the situation represents that the transmission environment is better than expected and a lower transmission power can be used to achieve the expected effect, therefore, the transmission power can be increased to the range not exceeding the working range for increasing the throughput. Although the increase of transmission power will cause interference to the other users, but the interference caused by the transmission power within the working range is anticipated and, in consequence, is acceptable. If the transmission power is within the working range [the maximum allowable transmission power, the minimum allowable transmission power], the situation represents that the communication is under control and the current data transmission rate is maintained.
Although the guideline of joint rate and power control has been specified in the 3GPP standard, to our best knowledge, however, it still remains an open issue for anyone in the field to develop his own proprietary adaptation algorithm. The current available algorithms provided in the literatures such as 3GPP TS 25.922, “Radio resource management strategies” V4.1.0, June 2001 or C. W. Sung etc, “Power Control and Rate Management for Wireless Multimedia CDMA System”, IEEE Trans. On Communication, Vol. 49, No. 7, pp. 1215-1226, July 2001, which centralize rate and power assignment and is difficult to realize because of the complexity of hardware design; and as Kin K. Leung etc, “Controlling QoS by Integrated Power Control and Link Adaptation in Broadband Wireless Network”, European Transaction On Communication, July-August Issue, 2000, which adjusts rate and power basing on the measurement of Signal to Noise Ratio (SNR) for each frame, however, when a SNR is huge, the SNR of the next frame might drop rapidly that causes the system to misinterpret the situation. In application, the method using a reference table is adopted for search solutions. Please refer to FIG. 2, which is a reference table currently used, wherein the horizontal line represents the strength of signal and the vertical line represents the throughput. Normally, the cellular phone will transmit a test signal first to the base station and, in return, the base station will transmit a feedback to the cellular phone according the SNR result of the test signal. The process is called close loop process. In accordance to the feedback and the reference table, the cellular phone can obtain several sets of processing gain and their corresponding throughputs, the same time, one set out of the obtained sets is adopted according to the current surroundings. For example, when the feedback SNR of the base station is 0, the cellular phone will choose a processing gain with higher throughput, thus, the processing gain=16 (as the —*—line seen in FIG. 2) and its corresponding throughput, and then adjust the throughput according to the information transmitted from the base station.
However, the current method has a major shortcoming, that is, a ping-pong effect will be incurred. When the variation of the environment is huge, the SNR will be affected and the throughput is instable that it may be raised as soon as it is decreased. The ping-pong effect is harmful to the data transmission. It will cause the loss of data packet and must be resolved.